The present invention relates to a device for controlling a continuously variable transmission and more particularly to a device for monitoring operation of the continuously variable transmission and releasing a lock-up device whenever an abnormal event takes place in a shift actuator system of the transmission.
Japanese patent application first publications Nos. 57-161359 and 57-161362 disclose a control device for a continuously variable transmission having a torque converter with a lock-up clutch, which Japanese publications correspond to the above mentioned U.S. patent application Ser. No. 361,491 now U.S. Pat. No. 4,510,822. According to this known control device, a shift valve is provided whereby when oil pressure in a drive pulley cylinder chamber is lower than a predetermined value, a lock-up device is released, while when the oil pressure is higher than the predetermined value, the lock-up device is engaged, for the purpose of releasing the lock-up device when a vehicle moves off from a standstill and accelerates rapidly. However, this control device is not provided with means for coping with a situation when abnormal event or trouble takes place in a shift actuator of the continuously variable transmission, so that if the shift actuator ceases its operation at a relatively small reduction ratio, the lock-up device is left engaged, causing the engine to stall, thereby disabling the vehicle running, or if the shift actuator ceases its operation at a relatively large reduction ratio, the engine will overrun, causing a damage on the engine.